Roller chains elongation, or chain growth, develops in service when material is worn off the pin and bushing bearing surfaces. Excessive chain growth may cause damage to the driving sprocket teeth. When the sprockets are in a vertical position chain sagging will be produced which in turn can cause the chain to fall away from the teeth of the lower sprocket. Chain manufacturers recommendations limit the useful life of a roller chain to wear elongation of 1.5% to 4% of the chain pitch.
It thus becomes necessary to be able to measure the wear of a chain in order to replace it before it causes a loss of driving power or permanent damage to the drive mechanism.
Wear elongation is commonly measured by laying the chain on a flat surface and measuring the spacing between a certain number of links with a ruler or a slide caliper. A rough assessment of the joint wear of a roller chain can be obtained by measuring its lateral deflection. None of the aforementioned methods provides a true measurement of the chain elongation under tension.